The internal bond test (Z-direction tensile test) explained

The bond strength between the layers of multi-layer materials (duplex or coated papers) is measured. This determines processability during printing or after coating. Loading in the test to TAPPI T 541 (or ISO 15754) is in the form of a force applied perpendicular to the surface of the specimen. To avoid the testing equipment influencing the separation of the layers, the force must also remain perpendicular to the specimen at failure, making high transverse stiffness and absolute parallelity of the specimen grips a primary essential.

Internal bond test with Zwick (Z-direction tensile test)

Specimen carriers arranged at 90° to each other enable 5 individual specimens to be tested in succession with a single application.

The specimen carriers are mounted on precision compression platens and are accordingly parallel and stiff in the tensile and transverse directions.

Specimen application: a specimen is attached to one specimen carrier using double-sided adhesive tape. Double-sided tape alone is applied to the opposing second carrier.

During the test sequence the specimen is initially pressed against the second specimen carrier. Compressive force and duration are freely adjustable.

The specimen is then separated in less than half a second in the tensile test.

The measured quantity is the tensile strength, related to the surface area of the specimen, giving the tensile stress.

Pressing on specimen plus tensile test in one operation.

Tests using half the test area are possible for high-strength specimens.

The test device is available in both manual and semi-automatic versions. The semi-automatic version allows a second strip of specimens to be prepared while the first strip is being tested.

Comparison with the Scott bond test

The internal bond test offers greatly reduced result scatter compared to the Scott bond test, which is used to determine similar paper properties. Other similar tests include ply-bond strength to DIN 54516 and the Scott bond test to TAPPI T 569, both of which apply peel loading to the specimen. A pendulum impact tester is used for the Scott bond test. The test therefore takes into account the high processing speeds usual for this material, but is considerably less reproducible due to internal energy losses through friction and vibration.